In U.S. Pat. Nos. 4,052,648 (Nola) and 4,266,177 (Nola), there are disclosed power factor controllers which are particularly useful in connection with inductive loads such as induction motors. These controllers, which sample the line voltage and current through the motor, include a thyristor which controls the power input to the motor in proportion to the detected phase difference between the sampled voltage and current, such that less power is provided to the motor in response to decreasing motor loading.
As is well understood in the art, a thyristor, i.e., an SCR or triac, will switch on if the gate electrode thereof is supplied with a current pulse whose duration may typically be of only a few microseconds, and will remain on until the anode current goes to a zero level. If the thyristor is used to control a sinusoidal current in a resistive load, the trigger pulse can be applied during any portion of the sine wave since the current will also be precisely in phase with the voltage. However, the current in an inductive load significantly lags the voltage and this can create problems in connection with triggering of the thyristor. More specifically, as is explained in more detail hereinbelow, if the firing or trigger pulse is generated at a time when current is flowing from the preceding half cycle (due to the current phase lag), the triac will already be turned on at that time. Further, when the current goes to zero and the triac goes off, the triac will remain off for an entire half cycle. Thus the trigger pulse will be without effect. The disadvantages of such operation in a thyristor control system are evident.
In a power factor controller of the type discussed above, this problem is avoided by supplying thyristor gate current using a fixed level signal rather than a trigger pulse. However, it will be appreciated that supplying a fixed level signal results in considerably more power consumption than supplying a trigger pulse. Moreover, in certain power factor controllers employing triacs, wherein the gate power is derived directly from the line voltage it has been found necessary to use a sensitive gate pilot triac to turn on the main power triac.